Contrasting provenance of Triassic/Jurassic sediments in North Sea Rift: a single zircon (SIMS), Sm–Nd and trace element study

Abstract

An ion-microprobe (SIMS) U–Pb zircon study has been performed on a selection of five samples in order to identify the various provenances of sands/sandstones from the Lunde and Statfjord Formations, the Gullfaks Oilfield. The data are integrated with Sr element analyses and new and published Sm–Nd whole rock data, which give consistent and additional information in this geologically complex area. Previous work has shown that the upper parts of the Lunde Formation have relatively low Nd model ages (typically 1.5 to 1.6 Ga), reflecting stable tectonic conditions. The overlying Raude and Eirikson Members of the Statfjord Formation record interfingering units recording relatively low (1.5 Ga) and substantially higher Nd model ages. This may reflect an early stage of rifting in the Northern North Sea Rift Zone, while the upper Nansen Member represents a return to relatively low, uniform Nd ages and quiet tectonic conditions. The present data reveal a more complex interplay between sediment provenance terranes than previously assumed. The sample of the Lunde Fm is characterised by a high number of zircon age peaks at 0.39 to 1.75 Ga. It was transported from the South and has an (ultimate) provenance in the SW margin of Norway after the mid-Devonian. The Eirikson #1 sample with t DM of 2.4 Ga, represents the income of units with substantially higher Nd model ages. A prominent peak of Archaean zircons at 2.56 to 3.00 Ga, reflects a sediment source in Archaean gneisses which were located in the North. Smaller peaks at 0.38 to 0.54 Ga and 1.10 to 1.90 Ga represent a separate provenance component in the East Shetland Platform. The Raude and Eirikson #2 samples have t DM of 1.7 and 1.8 Ga and show a cumulative zircon peak at 0.38 to 0.46 Ga and a high number of small peaks in the age ranges 0.70 to 2.23 Ga and 2.40 to 3.00 Ga. These units where transported from the South, and have an ultimate provenance in the East Shetland Platform. The sample of the Nansen Fm records a peak of zircon ages at 1.58 to 1.74 Ga, was derived from the South and represents re-cycled Devonian sediments from SW Norway. The four samples derived from the South, may have experienced sediment-redeposition. Of these, the Nansen sample is the best example of a sediment sequence, which has been preserved and transported as an isolated unit through erosion, sediment transport and re-deposition. There is a perfect match between the geochemistry of this sample and Devonian sediments from SW Norway, suggesting subordinate contamination from other provenance terranes during sedimentation and sediment re-deposition. This suggests that provenance studies of Devonian sediments from the North Sea area are important for an understanding of the ultimate source areas and processes of sediment re-deposition in the younger rift.